In an effort to meet stringent federal government emissions standards, engine systems may be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. Such EGR systems enable reduction in exhaust emissions while also improving fuel economy, especially at higher levels of engine boost.
One example of such an EGR system is illustrated by Duret in U.S. Pat. No. 6,135,088. Therein, a first inlet port of the engine cylinder is configured to deliver EGR while a second inlet port is configured to deliver fresh air, boosted by a compressor, to the cylinder. In this way, a low pressure EGR can be delivered to the cylinder.
However, Applicants have recognized that such EGR systems may have potential issues that can reduce overall EGR efficiency. As one example, it may be difficult to achieve high pressure EGR (HP-EGR), even if a turbine was added. As such, there may be engine operating conditions during which it may be advantageous to provide HP-EGR rather than, or in addition to, LP-EGR. Further still, since LP-EGR and HP-EGR require different control approaches, there may be delays in controlling both LP-EGR and HP-EGR, in particular, during transients.
Thus in one example, some of these issues may be at least partly addressed by a method of operating an engine cylinder comprising, recirculating a first amount of exhaust gas at a first pressure from a first exhaust passage of the cylinder to a first intake passage of the cylinder while recirculating a second amount of exhaust gas at a second, different pressure from a second, distinct exhaust passage of the cylinder via a second, distinct intake passage. In this way, both LP-EGR and HP-EGR may be simultaneously provided through distinct passages while being controlled independently. The EGR of different pressures may then be mixed and combusted in the cylinder.
For example, LP-EGR may be diverted from a first exhaust passage through a first EGR passage and delivered to an engine cylinder along a first intake passage, while HP-EGR may be diverted from a second exhaust passage through a second EGR passage and delivered to the engine cylinder along a second intake passage. The LP-EGR may be naturally aspirated and delivered to the engine cylinder at or below barometric pressure through a first intake valve of the first intake passage at a first, earlier intake valve timing, such as at the onset of an intake stroke. At the same time, HP-EGR may be delivered to the engine cylinder at compressor pressure through a second intake valve of the second intake passage at a second, later intake valve timing, such as after the intake stroke has begun. Specifically, the HP-EGR may be diverted from downstream of a turbine coupled only to the second exhaust passage to upstream of a compressor coupled only to the second intake passage, wherein the second intake passage is separate from the first intake passage. The HP-EGR and LP-EGR may be delivered separately to the engine cylinder and then mixed and combusted in the cylinder.
A timing of opening the first and second intake valves may be coordinated with a timing of opening of first and second exhaust valves coupled to the first and second exhaust passages, respectively. For example, higher pressure exhaust gases may be flowed through the turbine before release to the atmosphere or recirculated at higher pressure, while lower pressure exhaust gases may be directed to the atmosphere without flowing through the turbine or recirculated at or below barometric pressure. In addition to separating the HP-EGR from the LP-EGR, the staggered exhaust valve timings of the distinct exhaust valves increases the heat recovered from the exhaust gases. Also, by delivering the HP-EGR downstream of the compressor while delivering the LP-EGR through an intake passage not including the compressor, EGR may be advantageously kept out of the compressor, reducing compressor fouling and contamination issues. By operating LP-EGR separate from HP-EGR, they can also be independently controlled, reducing EGR control delays during transients. As such, EGR benefits can be availed with engine boost over a larger range of engine speed/load conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.